The invention is directed to connecting a string section with a pulling element, which extends through the string section, and to bracing string sections, in particular pipe sections, to form a string with a pulling element arranged between a counter support at a front end of the pipe string and an adaptor at a rear end thereof.
Corresponding systems are frequently used in trenchless pipe installation or renewal, for example, to brace the pipe string made of individual pipe sections and to connect the pipe string with a leading drilling or bursting tool with the least possible play.
Such system is described, for example, in DE 196 08 056 C1.
This document discloses a bursting tool which is pulled through an old pipe to be replaced. When the bursting tool advances through the old pipe, the old pipe is destroyed and a new pipe string is simultaneously pulled through the ground. The new pipe string to be pulled in includes individual short pipe sections.
To reduce friction during the pulling process, the pipe sections of the new pipe string have connecting sleeves with smooth outer and inner surfaces. However, these connecting sleeves are incapable of transmitting pulling forces, so that a tensioning device is provided for connecting the pipe string with the bursting tool with the least possible play.
The tensioning device acts on a cable or a chain which is secured to the bursting tool and routed through the pipe to be newly drawn-in.
The actual tensioning device, which is supported on the rear end of the last pipe section by an adapter, is located at the rear end of the pipe string.
The cable or chain is tensioned by means of hydraulic cylinders which are arranged perpendicular to the cable or the chain to account for the tight work space in the work shafts. The cable or the chain is hereby deflected by a freewheeling deflection pulley in the longitudinal direction of the hydraulic cylinders. The freewheel of the deflection pulley prevents the tensioned cable or the tensioned chain from rebounding, if several cylinder strokes are required for a complete tensioning operation.
The tensioning device described in DE 196 08 056 C1 has several disadvantages.
The deflection required by the perpendicular arrangement of the cylinders necessitates the use of flexible chains or cables as pulling member which undergo, however, a significant elongation, when subjected to high tension forces, which must be compensated by the tensioning device. This is disadvantageous, in particular, because the tensioning device must be released each time a new pipe section is added, so that at each tensioning cycle the expanded pulling member is unloaded and then again loaded.
Another disadvantage of the afore-described tensioning device is the considerable space requirement, in particular inside a reversing shaft, which is frequently used to start the operations to replace old pipes.
Tensioning devices, which use a cable or a chain as pulling member, have the additional disadvantage that the cable or the chain must typically have at least the same length as the overall length of the pipe string to be pulled in. In particular, at the start of the pipe replacement process, the pipe sections must be threaded onto the pulling member over a considerable distance.
DE 10 2004 033 566 B3 describes a device for bracing pipe sections with a pulling element to form a pipe string, wherein the device extends between a counter support at the front end of the pipe string and an adapter located at the rear end of the pipe string, and includes a linear tensioning drive for applying the tensioning force. The system is characterized by the parallel disposition of the linear tensioning drive in relation to the pipe string, wherein one side of the linear tensioning drive is supported on the adapter, while the other side is supported via at least one restraining element on the pulling element.
In this way, the linear tensioning drive can be arranged in part also inside the pipe string to be installed, thereby reducing the space requirement for the tensioning device.
However, to add a new pipe section to the pipe string, the tensioning device must be moved on the pulling element or released from the pulling element. This operation is executed by hand heretofore.